gdbserver: turn btrace-related target ops into methods

gdbserver: turn btrace-related target ops into methods

gdbserver/ChangeLog:
2020-02-20  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	Turn process_stratum_target's btrace-related ops (enable_btrace,
	disable_btrace, read_btrace, read_btrace_conf) into methods of
	process_target.

	* target.h (struct process_stratum_target): Remove the target ops.
	(class process_target): Add the target ops.
	(target_enable_btrace): Update.
	(target_disable_btrace): Update.
	(target_read_btrace): Update.
	(target_read_btrace_conf): Update.
	* target.cc (process_target::enable_btrace): Define.
	(process_target::disable_btrace): Define.
	(process_target::read_btrace): Define.
	(process_target::read_btrace_conf): Define.

	Update the derived classes and callers below.

	* linux-low.cc (linux_target_ops): Update.
	(linux_process_target:enable_btrace): Define as a wrapper around
	linux_enable_btrace.
	(linux_low_disable_btrace): Turn into ...
	(linux_process_target::disable_btrace): ... this.
	(linux_low_read_btrace): Turn into ...
	(linux_process_target::read_btrace): ... this.
	(linux_low_btrace_conf): Turn into ...
	(linux_process_target::read_btrace_conf): ... this.
	* linux-low.h (class linux_process_target): Update.
	* lynx-low.cc (lynx_target_ops): Update.
	* nto-low.cc (nto_target_ops): Update.
	* win32-low.cc (win32_target_ops): Update.